Mri apparatus with low-frequency cable integrated into the patient carrier

ABSTRACT

In a medical MRI apparatus, it is often desirable to have devices  62, 64  for communicating with or monitoring the patient  58  in the imaging volume  29  of the apparatus. Such devices need DC power or low-frequency connections with an area  70  outside the imaging volume. A device cable that does not interfere with the MRI magnetic fields is known per se. The invention proposes to fix the device cable in a groove  76  in the patient carrier  60  in such a manner that its strip-shaped conductors  78, 80  extend parallel to the field lines of the stationary field B of the apparatus. In this way, it is also possible to clip device carriers  72, 74  to the groove, thus supplying the devices with DC power or establishing an audio connection through the device cable in the groove.

The invention relates to a medical MRI apparatus provided with a patienttable having an elongated patient carrier the longitudinal direction ofwhich, if said patient carrier is placed in the imaging volume of theMRI apparatus, coincides with the direction of the stationary magneticfield in the imaging volume, which medical MRI apparatus is providedwith at least one device cable for low-frequency electric conduction,which device cable comprises mutually separated segments which are eachshorter than a predetermined value, and wherein the segments areseparated from each other by frequency-dependent separation elementsforming a conductor for low-frequency currents and an insulator forradio-frequency alternating current.

Such an MRI apparatus is known from the published European patentapplication No. 1 105 966. In this known MRI apparatus, a mobile patienttable can be used to take the patient to be examined up to the MRIapparatus. On the patient table there is a patient carrier on which thepatient lies. Next, the patient on the patient carrier is moved from thepatient table into the imaging volume of the apparatus. In the imagingvolume there is a device in the form of a television camera formonitoring the patient during recording the MRI image. The power supplyof the television camera takes place via a segmented device cablecomprised of segments which are each shorter than ¼ wavelength of theradio-frequency radiation generated in the MRI apparatus to produce theMRI image. The segments are separated from each other by self-inductanceelements which, as is known, form a conductor for DC current andlow-frequency signals and an insulator for high-frequency signals. Inthis case, low-frequency signals are to be taken to mean signals havinga frequency up to for example 20 kHz, so that they also include audiosignals, while high-frequency signals are to be taken to mean signalshaving a frequency above, for example, 20 MHz. In a typical MRIapparatus with a stationary field of for example 1.5 T, theradio-frequency signal has a frequency of approximately 64 MHz. Theself-inductance elements thus form an insulator for the high frequenciesand a conductor for the low frequencies. As a result, the cable thussegmented does not cause the antenna for emitting said high frequencies(referred to as RF body coil in an MRI apparatus) to becomenon-resonant, which would be the case if use were made of an unsegmentedcable and hence RF excitation of the tissue to be imaged would no longertake place, so that MRI imaging would be impossible.

It is possible that a patient has to be examined under conditions wherehe is surrounded by a large number of devices which all require electricpower and/or require low-frequency signal exchange with the environment.Apart from television cameras, said devices are, for example, devicesfor transferring physiologic quantities such as pulse beat, bloodpressure, temperature or ECG data to devices for communication, such asaudio communication with the patient, and to devices for illumination inthe imaging volume of the MRI apparatus. This would result in thepresence of a large number of device cables around and over the patient,which is inconvenient for the staff attending the patient and unpleasantfor the patient himself.

It is an object of the invention to provide a system enablingconnections to be set up to the devices to be used in the imagingvolume, such that disturbing influences on the operation of the MRIapparatus are precluded, and a plurality of device cables are notnecessary.

To achieve this, the MRI apparatus in accordance with the invention ischaracterized in that the device cable is rigidly connected throughoutits length to the patient carrier and extends in the longitudinaldirection of the patient carrier, and in that the device cable isembodied so as to be capable of being branched off. The longitudinaldirection of the patient carrier and hence the direction of the devicecable coincides with the direction of the stationary field of the MRIapparatus. As a result, any magnetic field originating from this cableextends perpendicularly to the stationary field of the apparatus, sothat no component is generated that disturbs this stationary field.Since the device cable is embodied so as to be capable of being branchedoff, it is achieved that each device to be used in the imaging volumecan be connected to the device cable at the location where it issituated during use. As a result, there are no device cables that extendfrom each device to a connection point outside the imaging volume.

In a preferred embodiment of the invention, the patient carrier isprovided with a groove in its longitudinal direction, which grooveaccommodates the device cable. It is thus achieved that the device cabledoes not have projections that might be inconvenient for the patientand/or the staff treating the patient.

In addition, the device cable can be readily fixed in a groove, so thatthe direction of this cable is always well-defined.

In a further preferred embodiment of the invention, the groove ispresent in a side edge of the patient carrier, such that the opening ofthe groove faces away from the patient carrier. This embodiment has theadvantage that the groove cannot easily be contaminated during operationwhile the accessibility thereof with a view to connecting the devices ismaintained.

In yet another preferred embodiment of the invention, the conductors inthe segments of the device cable take the form of strips, which stripsare provided so as to be in contact with the side walls of the groove.This embodiment has the advantage that the strips are fixed against thewalls of the groove, so that their positional definition is maximal, andthe branching-off for the devices can take place in a simple manner, forexample, by clamping a contact block in the groove at the end of thedevice connection cable.

In a different embodiment of the invention, the MRI device is providedwith a device which is to be arranged in or near the imaging volume,which device is provided with a connection cable which can be connectedto the device cable, which connection cable comprises mutually separatedsegments which are each shorter than a predetermined value, and whereinthe separation between the segments is brought about byfrequency-dependent separation elements forming a conductor forlow-frequency currents and an insulator for radio-frequency alternatingcurrent. This embodiment is advantageous if the device has acomparatively long connection cable or if the connection cable musttransfer a comparatively large current. In this case, a segmentedconnection cable does not electromagnetically disturb theelectromagnetic fields of the MRI apparatus, and also the connectioncable itself is not influenced by these fields.

In yet another embodiment of the invention, the device is arranged tomaintain a signal connection to an area outside the imaging volume via ahigh-frequency carrier wave. In this embodiment, signal-carryingconductors are not necessary as signal transfer is wireless, forexample, via a 2.4 GHz carrier wave. Such equipment is commerciallyavailable.

In a still further embodiment of the invention, the device is situatedin an envelope provided with a protective layer that provides shieldingagainst electromagnetic radiation from the device to the imaging volume.A number of devices to be used are provided with active electroniccomponents, such as said television camera. As an MRI apparatus is verysensitive to small field disturbances, these devices must not emitinterference fields. This effect is achieved by means of said shielding.

In a further embodiment of the invention, the protective layer is madeof a material having a resistivity below 0.05 Ωm, the protective layerhas a thickness below 40 μm, and the total surface area of theprotective layer is smaller than 100 cm².

In experiments it has been found that said values enable a goodshielding to be achieved while, in addition, this shielding does notbecome a source of disturbing eddy currents, which can be generated bythe gradient fields of the MRI apparatus.

In yet another embodiment of the invention, the frequency-dependentseparation elements are embodied so as to be bifilarly woundself-inductance elements.

The self-inductance elements (coils) can be oriented such that themagnetic field that they cause extends perpendicularly to the stationaryfield of the MRI apparatus, however, it is possible that a wrongorientation still leads to a component being generated in the directionof the stationary field. In the case of a bifilarly wound coil, anegligibly small magnetic field is caused, so that orientation errors donot cause the stationary field to be disturbed.

The invention will now be described with reference to the drawings. Inthe drawings:

FIG. 1 diagrammatically shows the general construction of a magneticresonance apparatus wherein the invention can be applied;

FIG. 2 is a more detailed view of the imaging volume of the magneticresonance apparatus in accordance with FIG. 1;

FIG. 3 is a cross-sectional view of the groove in the patient carrierwherein the device cable is provided.

The magnetic resonance apparatus (MRI apparatus) diagrammatically shownin FIG. 1 comprises a first magnetic system 1 for generating ahomogeneous stationary magnetic field B, a second magnetic system 3 forgenerating magnetic gradient fields, a power supply source 5 for thefirst magnetic system 1 and a power supply source 7 for the secondmagnetic system 3. A radio-frequency coil (RF coil) 9 is used togenerate a radio-frequency magnetic alternating field and is connected,for this purpose, to an RF transmission device with a radio-frequencysource 11. To detect electron spin resonance signals generated by theradio-frequency transmission field in an object to be examined (notshown), use can alternatively be made of the RF coil 9, which isconnected, for this purpose, to an RF receiving device comprising asignal amplifier 13. The output of the signal amplifier 13 is connectedto a detector circuit 15 that is connected to a central control device17. Said central control device 17 further controls a modulator 19 forthe RF source 11, the power supply source 7 and a monitor 21 for imagedisplay. A high-frequency oscillator 23 controls both the modulator 19and the detector 15 processing measuring signals. The forward andbackward RF signal traffic are separated from each other by a separationcircuit 14. For cooling the magnetic coils of the first magnetic systemI use is made of a cooling device 25 having coolant lines 27. The RFcoil 9 arranged within the magnetic systems 1 and 3 encloses an imagingvolume 29 which in the case of a device for producing images for medicalapplications is large enough to embrace a patient to be examined or apart of a patient to be examined, for example the head and the neck. Inthe imaging volume 29, a stationary magnetic field B, object-sectionsselecting gradient fields, and a spatially homogeneous RF alternatingfield can thus be generated. The RF coil 9 can combine the functions oftransmitting coil and measuring coil. For both functions, use canalternatively be made of different coils, for example of surface coilsas measuring coils. The assembly of coils 1, coil 9 and second magneticsystem (gradient coils) 3 is surrounded by a Faraday cage 31 thatprovides shielding from RF fields.

A power supply line 50-1 extends from the power supply source 7 to thefeedthrough device 30; also a power supply line 50-2 extends from thepower supply source 5 to the feedthrough device 30. The central controldevice 17 and the various parts to be controlled (not shown) of the MRIapparatus within the Faraday cage 31 are interconnected by means ofconnection lines 32 which are connected via the feedthrough device 30 tosaid parts to be controlled. In addition, an RF connection line 34 isprovided between the separation circuit 14 and the feedthrough device.Inside the Faraday cage, the power supply line 50-1 continues asconnection line 46-1, and the power supply line 50-2 continues asconnection line 46-2. The bundle of connection lines 32 is continuedwithin the Faraday cage as the bundle of connection lines 56.

FIG. 2 shows the imaging volume of the MRI apparatus of FIG. 1 ingreater detail. For the sake of clarity, only two coils of the firstmagnetic system 1 for generating a homogeneous stationary magnetic fieldB are shown. In the imaging volume 29, a patient 58 to be examined isplaced on a patient carrier 60 in such a manner that sectional images ofthe head and the neck can be produced. Within the imaging volume 29, orin the direct vicinity thereof, electrical connection equipment formaintaining a connection with the patient to be examined is present, inthis case a TV camera 62 and a lamp 64 for illuminating the field ofview to be recorded by the camera It is to be noted, however, that otherelectrical connection equipment can alternatively be provided in or nearthe imaging volume, such as sensors for recording blood pressure, heartbeat or brain activity of the patient, or for carrying out bidirectionalcommunication with the patient.

The TV camera 62 and the lamp 64 are supplied with power from supplyapparatus 70 via a respective supply conductor 66 and 68. The two supplyconductors 66 and 68 extend through the homogeneous magnetic field B andthrough the RF field generated by the coils 9. The present inventionprovides measures to preclude that the RF field generated by the coils 9and/or the homogeneous magnetic field B are disturbed such that thequality of the sectional images to be produced by means of the MRIapparatus are adversely affected. The devices 62 and 64 can each beattached to the patient carrier 60 via a device carrier 72, 74,respectively, in a manner which will be described in greater detail withreference to FIG. 3. To supply power to these devices 62 and 64, saiddevice carriers are each provided with a connection cable 66, 68,respectively, which extend through the preferably hollow devicecarriers. The connection cables electrically contact conductor strips ina groove 76 in the side face of the patient carrier 60, as will bedescribed in greater detail with reference to FIG. 3. The conductorstrips in groove 76 are connected to an interface unit 70 via aflexible, detachable cable. This interface unit may comprise a powersource for feeding said devices, and it may also comprise filteringmeans for separating a possible low-frequency signal, such as an audiosignal, from the DC power. Said measures for precluding disturbance ofthe MRI fields consist in that the connection cables 66 and 68 comprisemutually separated segments which are each shorter than a predeterminedvalue, i.e. shorter than ¼ of the wavelength of the RF field andpreferably shorter than {fraction (1/20)} of said wavelength, and inwhich the separation between the segments is brought about byfrequency-dependent separation elements forming a conductor forlow-frequency currents and an insulator for radio-frequency alternatingcurrent. These frequency-dependent separation elements are preferablyembodied so as to be bifilarly wound self-inductance elements. Saidsegments are embodied so as to be two twisted wires, as a result ofwhich the current flowing through these wires generates an unnoticeablemagnetic field outside the connection cable. Such connection cables areknown per se from said European patent application No. 1 105 966.

FIG. 3 is a cross-sectional view of the groove in the patient carrierwherein the device cable is provided. A groove 76 is provided in theside face of patient carrier 60 in such a manner that the opening ofthis groove faces sideways, i.e. faces away from the patient carrier.Strip-shaped conductors 78 and 80 of the device cable are provided inthe side walls of the groove. A device carrier 72 is detachably arrangedin the groove. The manner in which the device carrier 72 is provided andremoved is not relevant to the application of the invention; it isconceivable that the part 82 corresponding to the groove 76 is shiftedfrom a cavity (not shown in the drawing) to its position in a directionperpendicular to the plane of the drawing. Contact pads 84 and 86 areprovided in the side faces of the part 82, which contact padselectrically contact the strip-shaped conductors 80, 78, respectively,when the device carrier 72 is placed in the groove 76. The conductors 66of the connection cable 66 are attached to the contact pads 84 and 86,as a result of which power can be delivered to the device placed on thedevice carrier 72.

An envelope for an electronic device for use in MRI fields can bemanufactured from printed circuit board such as customarily used as asupport for electronic components. Such printed circuit board materialhas a thickness of, for example, 0.6 mm and is provided on one side witha layer of copper having a thickness of, for example, 17 μm. If acube-shaped envelope is desired, a surface composed of five squares isformed from the printed circuit board material. At the locations wherethe squares are interconnected, weakening grooves having a depth of 0.4mm are formed so as to enable the material to be folded more readilyinto a cube. At the angular points of the central square, circular holesare drilled so as to provide space during folding. Two additional holesare provided in the square that is to form the bottom of the cube, thusenabling feedthroughs for power supply and signal transfer to beprovided. The plate material thus shaped is folded into a cube that isopen on one side. In this process, the copper layer is maintained on theinside of the cube. After the open cube has been formed, a piece ofprinted circuit board material the size of which is equal to that of themissing side is formed, and this piece of material can be provided withelectronic components and soldered onto the open side. In this manner, ashielded envelope is obtained which, as shown in experiments, is capableof counteracting disturbance of the MRI fields in such a manner that noobservable image disturbance occurs, provided the shielding layer ismade of a material whose resistivity is below 0.05 Ωm, the layer has athickness below 40 μm, and the total surface area of the shielding layeris less than 100 cm².

1. A medical MRI apparatus provided with a patient table having anelongated patient carrier the longitudinal direction of which, if saidpatient carrier is placed in the imaging volume of the MRI apparatus,coincides with the direction of the stationary magnetic field b in theimaging volume, which medical MRI apparatus is provided with at leastone device cable for low-frequency electric conduction, which devicecable comprises mutually separated segments which are each shorter thana predetermined value, and wherein the segments are separated from eachother by frequency-dependent separation elements forming a conductor forlow-frequency currents and an insulator for radio-frequency alternatingcurrent, characterized in that the device cable is rigidly connectedthroughout its length to the patient carrier and extends in thelongitudinal direction of the patient carrier, and in that the devicecable is embodied so as to be capable of being branched off:
 2. Amedical MRI apparatus as claimed in claim 1, wherein the patient carrieris provided with a groove in its longitudinal direction, which grooveaccommodates the device cable.
 3. A medical MRI apparatus as claimed inclaim 2, wherein the groove is present in a side edge of the patientcarrier, such that the opening of the groove faces away from the patientcarrier.
 4. A medical MRI apparatus as claimed in claim 2, wherein theconductors in the segments of the device cable take the form of strips,which strips are provided so as to contact the side walls of the groove.5. A medical MRI apparatus as claimed in claim 1, provided with a devicewhich is to be arranged in or near the imaging volume, which device isprovided with a connection cable which can be connected to the devicecable, which connection cable comprises mutually separated segmentswhich are each shorter than a predetermined value, and wherein theseparation between the segments is brought about by frequency-dependentseparation elements forming a conductor for low-frequency currents andan insulator for radio-frequency alternating current.
 6. A medical MRIapparatus as claimed in claim 5, wherein the device is arranged tomaintain a signal connection to an area outside the imaging volume via ahigh-frequency carrier wave.
 7. A medical MRI apparatus as claimed inclaim 5, wherein the device is situated in an envelope provided with aprotective layer that provides shielding against electromagneticradiation from the device to the imaging volume.
 8. A medical MRIapparatus as claimed in claim 7, wherein the protective layer is made ofa material having a resistivity below 0.05 Ωm, wherein the protectivelayer has a thickness below 40 μm, and wherein the total surface area ofthe protective layer is smaller than 100 cm².
 9. A medical MRI apparatusas claimed in claim 1, wherein the frequency-dependent separationelements are embodied so as to be bifilarly wound self-inductanceelements.
 10. A patient carrier arranged for use in an MRI apparatus asclaimed claim 1.